Apparatus for measuring the distance of an airplane above the earth



Jan. 6, 1942. K. KUHNEMANN: 2,269,374 APPARATUS FOR MEASURING THE DISTANCE OF AN AIRPLANE ABOVE THE EARTH Fil ed A ril 26, 1940 INVENTOR.

ATTORNEYS I. Patented Jan. 1942 TANCE or an mm ABOVE THE Kari Kiihnemann, Beriin-Siemensstadt, Germany, assignor to Siemens Apparate and Maschinen Gesellschaft mit beschrinkter Haftnng, Berlin, Germany, a corporation of Germany Application April 26, 1940, Serial No. 331,892

' In Germany April 27, 1939 4 Claims. or. 171-352) This invention relates to means for measuring the distance of an object above the'earth, and

more-specifically to means for accurately determining the height of an aircraft above the ground by measuring the electrostatic capacity (capacitance) of a condenser formed by the craft, means carried thereby, and the ground, which capacitance varies in inverse proportion to the changes in altitudes of the aircraft.

Devices have previously been employed for determining aircraft altitudes by capacitance measurements, but such prior devices have involved the installation and employment of at least two conductors (condenser plates) attached to the wings or body of the aircraft, but insulated therefrom, and conformed tov the wing surfaces .in such a manner as not to offer any air resistance. The invention herein proposed and hereinafter described, by an original method of installing.

only one conductor (plate), and by an original method of providing for accurate measurements, will result in the marked advantage of increased simplicity of installation at a lower cost. The use of a single conductor will be a great advantage from a production viewpoint, inasmuch as the installation difl'lculties will be materially reduced. Furthermore, the proposed invention facilitatesjthe taking of altitude measurements, and will be highly accurate.

This proposed invention involves the use of the ac-called bridge method of measurement of electrostatic characteristics-in this case, capacitances-of the condenser formed by the aircraft and the ground, by a novel and original adapta- I tion of this well-known method.

One of the objects of the present invention is to provide a novel capacitance altimeter whichis light in weight and easy to install.

A further'object is to provide a novel capacitance apparatus for measuring the distance of an aircraft from the earth's surface, which apparatus is highly accurate.

Another object is to provide a simplified capaeitance device of the above character;

The above and additional objects and original features of this device will be more fully revealed in the following description and the appended drawing. The drawing, however, is sub- In the drawing, in which acters refer to like parts throughout,

Fig. 1 is a diagrammatic representation wit parts broken away of one embodiment of the invention mounted on an aircraft;

Figs. 2, 3 and 4 are schematic illustrations,

or wiring diagrams, representing progressive steps-for purposes of explanation-in capaci-- tance measurements by the bridge method, specifically as regards the modification of this method in the present invention; and,

I ing medium or dielectric between. The bodyof the plane constitutes yet another plate, so that considering from flat conductor on plane to earth, and from earth back to aircraft body proper, two condensers arethereby formed, connected with one another in.series,-the dielectric of both condensers being air. Since the capacitance of a condenser is an inverse function of the distance between the plates (such distance,

' in this case being the height of the craft above the ground) as the height is doubled, the capacitance is halved, and so on-it follows that a measurement of the capacitance of one or both of the condensers under consideration as the craft rises or descends will give an accurate index to the altitude of the aircraft.

Means for measuring the capacitance of these condensers is provided, and is one of the features of this invention. This means comprises a conventional bridge measuring device of four arms or branches, two of said arms l8 and I4 containing resistances or impedances of fixed and known electrical characteristics. A third arm contains a variable condenser I5, preferably a precision instrument such as a turning plate condenser, one terminal being connected to a terminal of the arm l3, and the other to the flat conductor in. The fourth and last arm of the bridge comprises a connection Ha between arm I and .the'aircraft body proper, the current flow in this arm travelling from aircraft body to earth, thence back to the flat conductor,,

thence to corner of thebridge where the leads of the receiver or galvanometer 20 and variable like reference char- 7 condenser II join. A screen l8, the purpom of which will be made clear iater, is provided for conductor plate It, and a suitable source of elec-' tromotive force is led to diametrically opposite comers of the bridge by connections I8 and II.

A telephone receiver, vibration galvanometer, or other suitable instrument for indicating the presence or absence of an electromotive force, is connected across bridge corners B and D (Fig. 2). Instrument 20 will hereinafter be referred to as the receiver.

when such a bridge isbalanced, that is. the variable condenser Ii is so adjusted that no sound is heard in the receiver, thus indicating zero diflerence of potential between corners B and D, the impedances in the four arms bear a definite mathematical ratio to'each other. Considering, forexampie, Fig. 2, and denoting the resistances of the two arms I! and I5 as Hi3 and R", respectively, and the capacitances v of the other two arms as Cab and CD0, then the change in capacitance of the variable condenser will also be a function of the change in altitude. The variable condenser may be provided with a scale, calibrated, not in electrical, but in lineal units (feet, yards, or meters) so that the altitude is indicated immediately the bridgeisbalanced.

The preceding paragraph sets forth the fundamental principle upon which the operations! this invention is based. For a detailed explanation ofth novel adaptation of this principle, consider, first, Fig. 2. Assume for the present, for purposes of simplicity only, that the screen i6 is not provided. Conductor i0 is attached to the aircraft body, but insulated therefrom. If

4 C1 denotes the capacitance of the condenser formed by this conductor and the earth, and C:

that of the condenser formed by the aircraft.

body and the earth, the two capacitances Cl and C2 are connected in series across the AB arm of the bridge as shown in the figure. Paralleling these is a third capacitance, denoted by Co,-

which is the capacitance of the condenser formed by conductor l0 and aircraft body ll directly the passage of a current, whereas a small capacitance oifers a great opposition. C: is large .enough, where increased or decreased, to permit a relatively free passage, and consequently the capacitance 01 will control "the current in the C1 C: branch of the arm AB. Disregarding C: then, the total capacitance of the arm AB is C1+Co, (two condensers in parallel). Since the quantity Co is independent of the altitude, the relative change of the total capacitance of the arm AB may therefore be expressed as A0; o i- 1 which, since C1 is relatively small, becomes This can be eifected by mounting the conductor I0 upon a screen It by way of suitably insulated brackets 30, shown in Fig. 5, which is larger-in area than the conductor, said screen being disposed. between conductor l0 and secured to the airplane body at a cutout wing portion thereof by way of suitably insulated brackets 38 shown in Fig. 5. The capacitance C4 of the condenser formed by the screen as one plate and the conductor Iii the other, which necessarily arises in this plan 0! mounting. i connected across the same bridge comers as the receiver. As these corners are points of zero potential when the bridge is balanced, 04 can consequently exert no influence as far as the balance is concerned, and will henceiorth be disregarded.

In case the area of screen II is twice the area of the conductor it, the ratio condition if no screen were used. This means that any change of 01 will exert a proportionately greater influence, and can consequently be more, accurately measured.

The capacitance C1 can also be materially reduced by installing the plate "I, not as a flat conductor, but as a rod. of more or less cylindrical'shape, partly surrounded by the screen.

Theintroduction o1 thescreen It creates a third condenser with the earth, this capacitance with each other and not by way of the earth-a condenser which is an inherent characteristic of this type of mounting. This capacitance, Co, is

fixed, and is relatively large on account of the 1 short distance between its plates. to capacitances C1 and C2, the capacitance C: is very great as compared to 01, since capacitances are a function of plate areas, and the area of the body H is large as compared with the area of the conductor ll. scends, both 01 and C: are aflected similarly, and

, both decrease or increase by the same percentage, but since Cr is so much smaller, it will exert the preponderant influence, and to such a degree that for allpractical purposes the quantity C: may be disregardedentireiy. This may best be understood ,if it is considered that a. very large capacitance oflers little or no opposition to As the plane rises or de- Returning being denoted by Ca. illustrates a mechanical embodiment of the three condensers C1, C: and Ca, and'l'ig. 3 their disposition in the bridge circuit, in the Y or star connection.

For purposeot' explanation, consider Fig. 4. Here is represented, not the actual connection of the condensers in the circuit (this is shown in Fig. 3), but a delta connection, or equivalent triangle, which is electrically equivalent to the star connection of Fig. 3. This transposition has been in accordance with a well-known transformation formula.

In Fig. 4, the capacitance C: .is parallel to the receiver, and C: is parallel to one resistance arm or the bridge, and both are accordingly ineffective. Onlythe capacitance C: is effective. However, it may be shown-that C: is substantially Fig. 1 atically v v 2,269,374 equal at all times to Cl, for, considering the or CagCr. .Therefore. the full capacitance change of the conductor Ill, is'eifective in the bridge circuit, and may be accurately measured by the bridge. What .is' claimed is:

1. In apparatus for determining the distance of an aircraft above that portion of the earths surface directly below the craft, an electrical bridge arrangement having two Joining arms thereof provided with resistor means,a variable condenser in a third arm of said bridge joining with a free end ofone of said first two arms, condenser means having one plate thereof defined by a conductor mounted on said craft and the other plate thereof by the earths surface directly below said craft, means for screening said conductor from said craft thus preventing the formation of another condenser between said responsive means, said variable condenser being manually operable an amount proportional to the altitude of said craft to balance said bridge. 2. In apparatus for determining the distance of an aircraft above that portion of the-earths responsive means connected across'another diagonal of said bridge whereby upon avariation in craft altitude said bridge is unbalanced causing current flow to said responsive means, said variable condenser being operable an amount proportional to the altitude of said craft to balance said bridge. a

3; In apparatus for determining the distance of an aircraft above that portion of. the earths surface directly below the craft, an electrical bridge arrangement having two joining arms thereof provided with resistor means, a variable condenser in a third arm of said bridge Joining with a .free end of one of said first two arms, condenser means having one plate thereof defined by a conductor mounted on said craft and the other plate thereof by the earths surface directly below said craft, a screen member extending about said .conductor and beyond the outer periphery of the conductor whereby another condenser is prevented from" being formed between said conductor and the body of-v said craft, said condenser means constitutinga'fourth arm of said brid e havingone end thereof connected to the free end of said third arm and the other end thereof with the free end'of the other of said first. two arms, a source of current connected across a diagonal of said bridge, and current flow responsivemeans connected across another diagonal of said bridge whereby upon a variation in craft altitude said bridge is unbalanced causing current flow to said responsive means, ;said variable condenser being operable an amount proportional to the altitude of said craft to balance said bridge.

4. In apparatus for determining the distance of an aircraft above that-portion of the earth's surface directly below the craft, .an electrical bridge arrangement having two Joining arms thereof provided with resistor means, a variable condenser in a third arm of saidbridge joinin with a free end of one of said first two arms, condenser means having one plate thereof deflned by a conductor mounted on said craft and ductor extending about and beyond the outer surface directly below the craft, an electrical bridge arrangement.having two joining arms thereof provided with resistor means, a variable condenser in a third arm of said bridge Joining with a free end of one of said first two arms, condenser means having one plate thereof defined by a conductor mounted on said craft and the other plate thereof by-the earths surface directly below said craft, a screenme'mber between said conductor and said craft whereby another condenser is prevented from being formed between said conductor and the body of said craft. said condenser means'constituting a fourth arm of said bridge having one end thereof connected to the free end of said third arm and the other end thereof with the free end of the other of said first two arms, a source of current connected across a diagonal of said bridge, and current flow periphery of said conductor whereby another condenser is prevented from forming between said conductor and the bodyof said craft, said condenser means constituting a fourth arm of said bridge having one end thereof connected to the free end of said third arm-and the other end thereof with the free end of the other of said first two arms, a source of current connected across a diagonal of said bridge, and current flow responsive means connected across an-.

other diagonal of said bridgewhereby upon a variation in craft altitude said bridge is unbalcraft to balance said bridge.

xiinnnmrm. 

